EP1738444A1 - Device for shifting frequency in an optical field with a pulsed laser source - Google Patents

Device for shifting frequency in an optical field with a pulsed laser source

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Publication number
EP1738444A1
EP1738444A1 EP05717003A EP05717003A EP1738444A1 EP 1738444 A1 EP1738444 A1 EP 1738444A1 EP 05717003 A EP05717003 A EP 05717003A EP 05717003 A EP05717003 A EP 05717003A EP 1738444 A1 EP1738444 A1 EP 1738444A1
Authority
EP
European Patent Office
Prior art keywords
optical
frequency
evolution
optical path
laser source
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Granted
Application number
EP05717003A
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German (de)
French (fr)
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EP1738444B1 (en
Inventor
Bertrand Baillon
Jean-Pierre Schlotterbeck
Alain Renard
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Thales SA
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Thales SA
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Application filed by Thales SA filed Critical Thales SA
Publication of EP1738444A1 publication Critical patent/EP1738444A1/en
Application granted granted Critical
Publication of EP1738444B1 publication Critical patent/EP1738444B1/en
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Classifications

    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S5/00—Semiconductor lasers
    • H01S5/06—Arrangements for controlling the laser output parameters, e.g. by operating on the active medium
    • H01S5/062—Arrangements for controlling the laser output parameters, e.g. by operating on the active medium by varying the potential of the electrodes
    • H01S5/06209—Arrangements for controlling the laser output parameters, e.g. by operating on the active medium by varying the potential of the electrodes in single-section lasers
    • H01S5/0622—Controlling the frequency of the radiation

Definitions

  • the present invention relates to a frequency shift device in an optical path with pulsed emission laser source.
  • a frequency variation Doppler effect
  • This frequency variation is generally determined by mixing the received signal with a signal generated by a local oscillator and affected by a frequency offset which is produced by a modulator of the electro-optical or acousto-optical type.
  • the present invention relates to a frequency shift device in an optical path with pulsed emission laser source, device which is as inexpensive as possible, compact, which is not a source of parasitic radiation, and which can be integrated in an architecture comprising optical fibers.
  • the device according to the invention comprises at least one frequency shift module containing an optical propagation medium, the optical path length of which is varied periodically, during each n-th laser beam emission pulse. of the desired frequency offset, with n> 1.
  • the optical propagation medium can be either an optical fiber, an optical waveguide, or an electro-optical crystal.
  • the evolution of the length of the optical path of the optical propagation medium is advantageously linear, and it is carried out by evolution, advantageously linear, during the pulses considered, of the stress applied to the optical fiber or to the waveguide or of the voltage applied to the electro-optical crystal.
  • FIG. 1 is a simplified block diagram of a Lidar of the prior art, of the type mentioned in the preamble
  • - Figure 2 is a simplified block diagram of a Lidar comprising a frequency shift circuit according to the invention
  • - Figure 3 is a block- simplified diagram of an example of a set of two frequency shift modules according to the invention.
  • the present invention is described below with reference to a Lidar, but it is understood that it is not limited to this single application, and that it can be implemented in various fields in which it is necessary to produce a frequency shift on high frequencies such as frequencies above a few Ghz.
  • the block diagram of FIG. 1 shows a laser source
  • a first output of the coupler 2 corresponding to the “signal” channel, is connected, by means of a pulse-cutting element 1A (also known by the name of “shutter”, and which can be of the electro- optical) to an acousto-optical modulator 3, followed by an amplifier 4.
  • the amplifier 4 is connected to a first terminal of a coupler 5 with polarization separation.
  • An output of the coupler 5 is connected to a Lidar beam transmission-reception system, referenced 6 as a whole.
  • the second output of the coupler 2 is connected to a first input of a polarization maintaining coupler 7, the other input of which is connected to the coupler 5.
  • the output of the coupler 7 is connected to a mixing and detection unit 8, followed by a filter circuit 9 and signal processing circuits 10.
  • a mixing and detection unit 8 followed by a filter circuit 9 and signal processing circuits 10.
  • the modulator 3 of FIG. 1 has been replaced by a device 11 for frequency shifting.
  • the other elements of the system of FIG. 2 can be the same as those of FIG. 1.
  • the device 11 can comprise one or more elementary frequency shift modules. Each of these elementary modules includes an optical propagation medium such as an optical fiber, an optical waveguide or an electro-optical crystal.
  • this support acts on this support so as to vary it periodically, during each n-th laser beam emission pulse, the optical path length as a function of the desired frequency offset, with n ⁇ 1.
  • This variation in path length corresponds to a variation in the refractive index of the optical medium.
  • this support is an optical fiber or a waveguide
  • this variation can be obtained by imposing a mechanical stress on the optical support.
  • it is an electro-optical crystal, the variation is obtained by applying an appropriate electrical voltage to its electrodes.
  • the electro-optical crystal then does not operate in a conventional electro-optical modulator, but in a device causing a shift, fixed or variable, in the frequency of the pulsed laser beam which it receives.
  • the evolution of the mechanical stress or of the electric tension is, in the present case, linear as a function of time.
  • This linear evolution generates a constant Doppler shift, and consequently a fixed frequency shift between the “signal” and “local oscillator” channels, which allows coherent heterodyne detection.
  • This Doppler shift is imposed on the “signal” channel into which the device 11 is inserted (as shown in solid lines in FIG. 2), and / or in the “local oscillator” channel (the second device with shift modules is then referenced 11A and drawn in broken lines in FIG. 2).
  • a variation in the length of the optical path is generated when the speed of the beam passing through the device 11 (and / or 11 A) varies.
  • the laser beam is passed two or more times through the shifting device.
  • the first reason is economic: if the applied voltage slope is not sufficient to obtain the desired offset over the duration of the pulse, the beam is passed through the module again to reach the desired final offset.
  • the second reason is that the shift module could very well be located behind the coupler 5, just before the emission of the beam into the atmosphere, in which case the backscattered beam could itself undergo a shift (identical or inverse or other than the initial offset).
  • the frequency offset generated by the device 11 (and / or 11A) can be controlled using the value of the offset measurement at the output of this offset device.
  • the electro-optical control voltage or the mechanical stress applied to the optical propagation medium of this device can then be slaved to a fixed value or evolving according to a determined law which is a function of the application in question.
  • This frequency offset measurement can be used to determine a noise level or in post-processing performed in signal processing circuits of the backscattered signal.
  • the invention is also applicable when blending in SSB (in phase and in phase quadrature) is carried out.
  • the device 11 (and / or 11 A) is provided with two identical shift modules 12, 13, connected in parallel, and followed by a switch 14.
  • the device 11 may comprise more than two modules.
  • the shutter 1A is controlled so as to regularly cut the incident continuous laser beam and thus produce regularly spaced pulses.
  • a frequency shift is effected periodically for each n-th pulse (n> 1), that is to say that the successive pulses are grouped in successive series of n pulses each, and the first pulse of each series is processed by applying said evolution law to it, these series each comprising at least one pulse.
  • the pulses passing through the module 13 may or may not be synchronous with those passing through the module 12. In the case illustrated in FIG. 3, the pulses passing through the module 13 are offset in time (using means known per se and not shown) with respect to those passing through the module 12.
  • the law of evolution applied to the pulses in the module 13 may or may not be different from that applied in the module 12.
  • the switch 14 is controlled so as to collect the output beam from one or the other module, for example as a function of the nature of the targets targeted by a Lidar equipped with the device of the invention. According to a variant of the invention, a single shift module is used, the switch 14 then being deleted.
  • the applications of the device of the invention are numerous: in addition to the application considered here at telemetry and velocimetry Lidars, with, where appropriate “chirped” beams, mention will be made of telecommunications (multiplexers and demultiplexers, for example), reduction of Brillouin effects in optical waveguides, etc.
  • the device of the invention thanks to the replacement of the conventional acousto-optical or electro-optical modulator by an offset device with variation in optical path length, makes it possible to avoid the emission of annoying harmonic lines, and to avoid the production of a radio frequency modulation.
  • the mechanical or electrical stresses involved in this shifting device are low and therefore easy to produce.
  • the electro-optical technologies used by the invention come from those commonly used in telecommunications, the components used being compact and inexpensive, since they are produced in integrated technique. The costs of these components can be further reduced by hybridizing them on a planar waveguide (in integrated optics technology), which reduces the number of connections by wires and optical fibers.

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  • Physics & Mathematics (AREA)
  • Condensed Matter Physics & Semiconductors (AREA)
  • General Physics & Mathematics (AREA)
  • Electromagnetism (AREA)
  • Optics & Photonics (AREA)
  • Optical Radar Systems And Details Thereof (AREA)
  • Optical Modulation, Optical Deflection, Nonlinear Optics, Optical Demodulation, Optical Logic Elements (AREA)
  • Optical Integrated Circuits (AREA)

Abstract

The invention relates to a device for shifting frequency in an optical path with a pulsed-emission laser source. The inventive device is characterised in that it comprises at least one frequency shifting module (12, 13) containing an optical propagation medium having an optical path length which is modified as a function of the desired frequency shift during at least one pulse selected from n consecutive laser emission pulses, where n=1. The invention is suitable for lidars.

Description

DISPOSITIF DE DECALAGE DE FREQUENCE DANS UN CHEMIN OPTIQUE A SOURCE LASER PULSEE FREQUENCY SHIFT DEVICE IN AN OPTICAL PATH WITH PULSED LASER SOURCE
La présente invention se rapporte à un dispositif de décalage de fréquence dans un chemin optique à source laser à émission puisée. Dans des dispositifs comportant une source laser et une détection cohérente hétérodyne, il peut être nécessaire de déterminer une variation de fréquence (effet Doppler) se produisant sur une partie du trajet du faisceau laser et résultant d'un phénomène physique que l'on désire compenser ou mesurer. Ceci est en particulier le cas des Lidars servant à mesurer la vitesse et la direction du vent par rétrodiffusion du faisceau laser sur des aérosols portés par le vent. Cette variation de fréquence est généralement déterminée par mélange du signal reçu avec un signal généré par un oscillateur local et affecté d'un décalage de fréquence qui est produit par un modulateur de type électro-optique ou acousto-optique. Un tel dispositif modulateur est cher, volumineux, peu discret, car il rayonne en haute fréquence (présence de raies harmoniques de la fréquence fondamentale du signal, susceptibles de perturber le traitement de signal effectué en aval, ce qui risque de limiter la précision de la mesure de vitesse et de rétrécir la fenêtre de mesure spectrale ). La présente invention a pour objet un dispositif de décalage de fréquence dans un chemin optique à source laser à émission puisée, dispositif qui soit le moins onéreux possible, peu encombrant, qui ne soit pas une source de rayonnements parasites, et qui puisse s'intégrer dans une architecture comportant des fibres optiques. Le dispositif conforme à l'invention comporte au moins un module de décalage en fréquence contenant un support de propagation optique dont on fait varier périodiquement, au cours de chaque n-ième impulsion d'émission de faisceau laser, la longueur de chemin optique en fonction du décalage de fréquence désiré, avec n>1 . Le support de propagation optique peut être soit une fibre optique, soit un guide d'ondes optique, soit un cristal électro- optique. Dans le cas où le système dans lequel on fait varier la fréquence est un Lidar, l'évolution de la longueur du chemin optique du support de propagation optique est avantageusement linéaire, et elle est réalisée par évolution, avantageusement linéaire, au cours des impulsions considérées, de la contrainte appliquée à la fibre optique ou au guide d'ondes ou de la tension appliquée au cristal électro-optique. La présente invention sera mieux comprise à la lecture de la description détaillée d'un mode de réalisation, pris à titre d'exemple non limitatif et illustré par le dessin annexé, sur lequel : - la figure 1 est un bloc-diagramme simplifié d'un Lidar de l'art antérieur, du type cité en préambule, - la figure 2 est un bloc-diagramme simplifié d'un Lidar comportant un circuit de décalage de fréquence conforme à l'invention, et - la figure 3 est un bloc-diagramme simplifié d'un exemple d'un ensemble de deux modules de décalage de fréquence conforme à l'invention. La présente invention est décrite ci-dessous en référence à un Lidar, mais il est bien entendu qu'elle n'est pas limitée à cette seule application, et qu'elle peut être mise en œuvre dans divers domaines dans lesquels on a besoin de produire un décalage en fréquence sur des fréquences élevées telles que les fréquences supérieures à quelques Ghz. On a représenté sur le bloc-diagramme de la figure 1 une source laserThe present invention relates to a frequency shift device in an optical path with pulsed emission laser source. In devices comprising a laser source and a heterodyne coherent detection, it may be necessary to determine a frequency variation (Doppler effect) occurring over part of the path of the laser beam and resulting from a physical phenomenon which it is desired to compensate or measure. This is in particular the case of Lidars used to measure the speed and direction of the wind by backscattering of the laser beam on aerosols carried by the wind. This frequency variation is generally determined by mixing the received signal with a signal generated by a local oscillator and affected by a frequency offset which is produced by a modulator of the electro-optical or acousto-optical type. Such a modulator device is expensive, bulky, not very discreet, because it radiates at high frequency (presence of harmonic lines of the fundamental frequency of the signal, capable of disturbing the signal processing carried out downstream, which risks limiting the accuracy of the speed measurement and shrink the spectral measurement window). The present invention relates to a frequency shift device in an optical path with pulsed emission laser source, device which is as inexpensive as possible, compact, which is not a source of parasitic radiation, and which can be integrated in an architecture comprising optical fibers. The device according to the invention comprises at least one frequency shift module containing an optical propagation medium, the optical path length of which is varied periodically, during each n-th laser beam emission pulse. of the desired frequency offset, with n> 1. The optical propagation medium can be either an optical fiber, an optical waveguide, or an electro-optical crystal. In the case where the system in which the frequency is varied is a Lidar, the evolution of the length of the optical path of the optical propagation medium is advantageously linear, and it is carried out by evolution, advantageously linear, during the pulses considered, of the stress applied to the optical fiber or to the waveguide or of the voltage applied to the electro-optical crystal. The present invention will be better understood on reading the detailed description of an embodiment, taken by way of nonlimiting example and illustrated by the appended drawing, in which: - Figure 1 is a simplified block diagram of a Lidar of the prior art, of the type mentioned in the preamble, - Figure 2 is a simplified block diagram of a Lidar comprising a frequency shift circuit according to the invention, and - Figure 3 is a block- simplified diagram of an example of a set of two frequency shift modules according to the invention. The present invention is described below with reference to a Lidar, but it is understood that it is not limited to this single application, and that it can be implemented in various fields in which it is necessary to produce a frequency shift on high frequencies such as frequencies above a few Ghz. The block diagram of FIG. 1 shows a laser source
1 reliée à une entrée d'un coupleur 2 à maintien dé polarisation. Une première sortie du coupleur 2, correspondant à la voie « signal », est reliée, par l'intermédiaire d'un élément 1A de découpage impulsionnel (connu également sous l'appellation de « shutter », et qui peut être de type électro- optique) à un modulateur acousto-optique 3, suivi d'un amplificateur 4. L'amplificateur 4 est relié à une première borne d'un coupleur 5 à séparation de polarisations. Une sortie du coupleur 5 est reliée à un système d'émission-réception de faisceaux Lidar, référencé 6 dans son ensemble. La deuxième sortie du coupleur 2 est reliée à une première entrée d'un coupleur à maintien de polarisation 7, dont l'autre entrée est reliée au coupleur 5. La sortie du coupleur 7 est reliée à une unité de mélange et de détection 8, suivie d'un circuit de filtrage 9 et de circuits 10 de traitement de signal. Dans le bloc-diagramme de la figure 2, les éléments similaires à ceux de la figure 1 sont affectés des mêmes références numériques. La différence essentielle entre les systèmes des figures 1 et 2 est que le modulateur 3 de la figure 1 a été remplacé par un dispositif 11 de décalage de fréquence. Les autres éléments du système de la figure 2 peuvent être les mêmes que ceux de la figure 1. Le dispositif 11 peut comporter un ou plusieurs modules élémentaires de décalage de fréquence. Chacun de ces modules élémentaires comporte un support de propagation optique tel qu'une fibre optique, un guide d'ondes optique ou un cristal électro-optique. On agit sur ce support de façon à en faire varier périodiquement, au cours de chaque n-ième impulsion d'émission de faisceau laser, la longueur de chemin optique en fonction du décalage de fréquence désiré, avec n≥1 . Cette variation de longueur de trajet correspond à une variation d'indice de réfraction du support optique. Dans le cas où ce support est une fibre optique ou un guide d'ondes, cette variation peut être obtenue en imposant une contrainte mécanique au support optique. S'il s'agit d'un cristal électro-optique, la variation est obtenue en appliquant une tension électrique appropriée à ses électrodes. A ce propos, il est important de noter que le cristal électro-optique ne fonctionne alors pas en modulateur électro-optique classique, mais en dispositif provoquant un décalage, fixe ou variable, de la fréquence du faisceau laser puisé qu'il reçoit. Dans chaque module, l'évolution de la contrainte mécanique ou de la tension électrique est, dans le cas présent, linéaire en fonction du temps. Cette évolution linéaire engendre un décalage Doppler constant, et par conséquent un décalage fixe de fréquence entre les voies « signal » et « oscillateur local », ce qui permet la détection cohérente hétérodyne. Ce décalage Doppler est imposé à la voie « signal » dans laquelle est inséré le dispositif 11 (comme représenté en traits continus sur la figure 2), et/ou dans la voie « oscillateur local » (le deuxième dispositif à modules de décalage est alors référencé 11A et dessiné en traits interrompus sur la figure 2). Une variation de la longueur du chemin optique est engendrée lorsque la vitesse du faisceau traversant le dispositif 11 (et/ou 11 A) varie. Cette vitesse varie de manière inversement proportionnelle à l'indice de réfraction du milieu traversé. Selon une variante de l'invention, on fait passer deux ou plusieurs fois le faisceau laser dans le dispositif de décalage. La première raison est d'ordre économique : si la pente en tension appliquée n'est pas suffisante pour obtenir le décalage voulu sur la durée de l'impulsion, on fait traverser une nouvelle fois au faisceau le module pour atteindre le décalage final désiré. La deuxième raison est que le module de décalage pourrait très bien être situé derrière le coupleur 5, juste avant l'émission du faisceau dans l'atmosphère, auquel cas le faisceau rétrodiffusé pourrait subir lui-même un décalage (identique ou inverse ou autre que le décalage initial). Le décalage de fréquence généré par le dispositif 11 (et/ou 11A) peut être contrôlé à l'aide de la valeur de la mesure du décalage à la sortie de ce dispositif de décalage. La tension de commande électro-optique ou la contrainte mécanique appliquée au support de propagation optique de ce dispositif (donc la loi d'évolution du décalage de fréquence) peut alors être asservie à une valeur fixe ou évoluant selon une loi déterminée qui est fonction de l'application considérée. Cette mesure de décalage de fréquence peut être utilisée pour déterminer un niveau de bruit ou dans le post-traitement effectué dans les circuits de traitement de signal du signal rétro-diffusé. L'invention est également applicable lorsque l'on effectue des mélanges en BLU (en phase et en quadrature de phase). Selon un aspect avantageux de l'invention, et comme représenté en figure 3, on munit le dispositif 11 (et/ou 11 A) de deux modules de décalage identiques 12, 13, branchés en parallèle, et suivis d'un commutateur 14. Il est bien entendu que le dispositif 11 (et/ou '11 A) peut comporter plus de deux modules. Le shutter 1A est commandé de façon à découper régulièrement le faisceau laser continu incident et produire ainsi des impulsions régulièrement espacées. Selon l'invention, dans chaque module de décalage 12, 13, on opère périodiquement un décalage de fréquence pour chaque n-ième impulsion (n>1 ), c'est-à-dire que les impulsions successives sont groupées en séries successives de n impulsions chacune, et on traite la première impulsion de chaque série en lui appliquant ladite loi d'évolution, ces séries comportant chacune au moins une impulsion. Les impulsions passant dans le module 13 peuvent être synchrones ou non de celles passant dans le module 12. Dans le cas illustré en figure 3, les impulsions passant dans le module 13 sont décalées dans le temps (à l'aide de moyens connus en soi et non représentés) par rapport à celles passant dans le module 12. La loi d'évolution appliquée aux impulsions dans le module 13 peut être différente ou non de celle appliquée dans le module 12. Le commutateur 14 est commandé de façon à recueillir le faisceau de sortie de l'un ou l'autre module, par exemple en fonction de la nature des cibles visées par un Lidar équipé du dispositif de l'invention. Selon une variante de l'invention, on utilise un seul module de décalage, le commutateur 14 étant alors supprimé. Les applications du dispositif de l'invention sont nombreuses : outre l'application considérée ici aux Lidars de télémétrie et de vélocimétrie, avec, le cas échéant des faisceaux « chirpés », on citera les télécommunications (multiplexeurs et démultiplexeurs, par exemple), la réduction des effets Brillouin dans les guides d'ondes optiques, etc. Le dispositif de l'invention, grâce au remplacement du modulateur acousto-optique ou électro-optique classique par un dispositif de décalage à variation de longueur de parcours optique, permet d'éviter l'émission de raies harmoniques gênantes, et d'éviter la production d'une modulation radiofréquence. Les contraintes mécaniques ou électriques mises en jeu dans ce dispositif de décalage sont faibles et donc faciles à produire. Les technologies électro-optiques employées par l'invention sont issues de celles couramment employées en télécommunications, les composants utilisés étant compacts et peu coûteux, car réalisés en technique intégrée. On peut encore réduire les coûts de ces composants en les hybridant sur un guide d'ondes planaire (en technologie d'optique intégrée), ce qui réduit le nombre de connexions par fils et fibres optiques. 1 connected to an input of a coupler 2 with polarization maintenance. A first output of the coupler 2, corresponding to the “signal” channel, is connected, by means of a pulse-cutting element 1A (also known by the name of “shutter”, and which can be of the electro- optical) to an acousto-optical modulator 3, followed by an amplifier 4. The amplifier 4 is connected to a first terminal of a coupler 5 with polarization separation. An output of the coupler 5 is connected to a Lidar beam transmission-reception system, referenced 6 as a whole. The second output of the coupler 2 is connected to a first input of a polarization maintaining coupler 7, the other input of which is connected to the coupler 5. The output of the coupler 7 is connected to a mixing and detection unit 8, followed by a filter circuit 9 and signal processing circuits 10. In the block diagram of FIG. 2, the elements similar to those of FIG. 1 are given the same reference numbers. The difference essential between the systems of FIGS. 1 and 2 is that the modulator 3 of FIG. 1 has been replaced by a device 11 for frequency shifting. The other elements of the system of FIG. 2 can be the same as those of FIG. 1. The device 11 can comprise one or more elementary frequency shift modules. Each of these elementary modules includes an optical propagation medium such as an optical fiber, an optical waveguide or an electro-optical crystal. One acts on this support so as to vary it periodically, during each n-th laser beam emission pulse, the optical path length as a function of the desired frequency offset, with n≥1. This variation in path length corresponds to a variation in the refractive index of the optical medium. In the case where this support is an optical fiber or a waveguide, this variation can be obtained by imposing a mechanical stress on the optical support. If it is an electro-optical crystal, the variation is obtained by applying an appropriate electrical voltage to its electrodes. In this regard, it is important to note that the electro-optical crystal then does not operate in a conventional electro-optical modulator, but in a device causing a shift, fixed or variable, in the frequency of the pulsed laser beam which it receives. In each module, the evolution of the mechanical stress or of the electric tension is, in the present case, linear as a function of time. This linear evolution generates a constant Doppler shift, and consequently a fixed frequency shift between the “signal” and “local oscillator” channels, which allows coherent heterodyne detection. This Doppler shift is imposed on the “signal” channel into which the device 11 is inserted (as shown in solid lines in FIG. 2), and / or in the “local oscillator” channel (the second device with shift modules is then referenced 11A and drawn in broken lines in FIG. 2). A variation in the length of the optical path is generated when the speed of the beam passing through the device 11 (and / or 11 A) varies. This speed varies inversely proportional to the refractive index of the medium crossed. According to a variant of the invention, the laser beam is passed two or more times through the shifting device. The first reason is economic: if the applied voltage slope is not sufficient to obtain the desired offset over the duration of the pulse, the beam is passed through the module again to reach the desired final offset. The second reason is that the shift module could very well be located behind the coupler 5, just before the emission of the beam into the atmosphere, in which case the backscattered beam could itself undergo a shift (identical or inverse or other than the initial offset). The frequency offset generated by the device 11 (and / or 11A) can be controlled using the value of the offset measurement at the output of this offset device. The electro-optical control voltage or the mechanical stress applied to the optical propagation medium of this device (therefore the law of evolution of the frequency offset) can then be slaved to a fixed value or evolving according to a determined law which is a function of the application in question. This frequency offset measurement can be used to determine a noise level or in post-processing performed in signal processing circuits of the backscattered signal. The invention is also applicable when blending in SSB (in phase and in phase quadrature) is carried out. According to an advantageous aspect of the invention, and as shown in FIG. 3, the device 11 (and / or 11 A) is provided with two identical shift modules 12, 13, connected in parallel, and followed by a switch 14. It is understood that the device 11 (and / or '11 A) may comprise more than two modules. The shutter 1A is controlled so as to regularly cut the incident continuous laser beam and thus produce regularly spaced pulses. According to the invention, in each shift module 12, 13, a frequency shift is effected periodically for each n-th pulse (n> 1), that is to say that the successive pulses are grouped in successive series of n pulses each, and the first pulse of each series is processed by applying said evolution law to it, these series each comprising at least one pulse. The pulses passing through the module 13 may or may not be synchronous with those passing through the module 12. In the case illustrated in FIG. 3, the pulses passing through the module 13 are offset in time (using means known per se and not shown) with respect to those passing through the module 12. The law of evolution applied to the pulses in the module 13 may or may not be different from that applied in the module 12. The switch 14 is controlled so as to collect the output beam from one or the other module, for example as a function of the nature of the targets targeted by a Lidar equipped with the device of the invention. According to a variant of the invention, a single shift module is used, the switch 14 then being deleted. The applications of the device of the invention are numerous: in addition to the application considered here at telemetry and velocimetry Lidars, with, where appropriate “chirped” beams, mention will be made of telecommunications (multiplexers and demultiplexers, for example), reduction of Brillouin effects in optical waveguides, etc. The device of the invention, thanks to the replacement of the conventional acousto-optical or electro-optical modulator by an offset device with variation in optical path length, makes it possible to avoid the emission of annoying harmonic lines, and to avoid the production of a radio frequency modulation. The mechanical or electrical stresses involved in this shifting device are low and therefore easy to produce. The electro-optical technologies used by the invention come from those commonly used in telecommunications, the components used being compact and inexpensive, since they are produced in integrated technique. The costs of these components can be further reduced by hybridizing them on a planar waveguide (in integrated optics technology), which reduces the number of connections by wires and optical fibers.

Claims

REVENDICATIONS
1. Dispositif de décalage de fréquence (11, 11 A) dans un chemin optique à source laser à émission puisée, caractérisé en ce qu'il comporte au moins un module de décalage en fréquence (12, 13) contenant un support de propagation optique dont on fait varier périodiquement, au cours de chaque n-ième impulsion d'émission de faisceau laser la longueur de chemin optique en fonction du décalage de fréquence désiré, avec n>1.1. Frequency shift device (11, 11 A) in an optical path with pulsed emission laser source, characterized in that it comprises at least one frequency shift module (12, 13) containing an optical propagation medium whose optical path length is varied periodically, during each n-th laser beam emission pulse as a function of the desired frequency offset, with n> 1.
2. Dispositif selon la revendication 1, caractérisé en ce que le support de propagation optique est l'un des dispositifs suivants : une fibre optique, un guide d'ondes ou un cristal électro-optique.2. Device according to claim 1, characterized in that the optical propagation medium is one of the following devices: an optical fiber, a waveguide or an electro-optical crystal.
3. Dispositif selon la revendication 1 ou 2, caractérisé en ce que le système dans lequel on fait varier la fréquence est un Lidar et que l'évolution de la longueur du chemin optique du support de propagation optique est linéaire.3. Device according to claim 1 or 2, characterized in that the system in which the frequency is varied is a Lidar and that the evolution of the length of the optical path of the optical propagation medium is linear.
4. Dispositif selon la revendication 3, caractérisé en ce que les lois d'évolution sont égales d'une impulsion choisie à la suivante. 4. Device according to claim 3, characterized in that the laws of evolution are equal from one selected pulse to the next.
5. Dispositif selon la revendication 3, caractérisé en ce que les lois d'évolution sont différentes d'une impulsion à la suivante.5. Device according to claim 3, characterized in that the laws of evolution are different from one impulse to the next.
6. Dispositif selon l'une des revendications précédentes, caractérisé en ce que l'on branche en parallèle au moins deux modules de décalage de fréquence (12,13), un commutateur (14) étant branché aux sorties de ces modules et commandé pour recueillir le faisceau optique de sortie de l'un ou l'autre module.6. Device according to one of the preceding claims, characterized in that at least two frequency shift modules (12, 13) are connected in parallel, a switch (14) being connected to the outputs of these modules and controlled for collect the output optical beam from either module.
7. Dispositif , selon l'une des revendications précédentes, caractérisé en ce que la loi d'évolution du décalage de fréquence est asservie à une valeur fixe.7. Device according to one of the preceding claims, characterized in that the law of evolution of the frequency offset is slaved to a fixed value.
8. Dispositif selon l'une des revendications 1 à 6, caractérisé en ce que la loi d'évolution du décalage de fréquence est asservie à une valeur variable. 8. Device according to one of claims 1 to 6, characterized in that the law of evolution of the frequency offset is controlled by a variable value.
EP05717003A 2004-03-12 2005-03-11 Device for shifting frequency in an optical field with a pulsed laser source Active EP1738444B1 (en)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
FR0402599A FR2867619B1 (en) 2004-03-12 2004-03-12 FREQUENCY OFFSET DEVICE IN A PULSED LASER SOURCE OPTICAL PATH
PCT/EP2005/051107 WO2005099052A1 (en) 2004-03-12 2005-03-11 Device for shifting frequency in an optical field with a pulsed laser source

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EP1738444A1 true EP1738444A1 (en) 2007-01-03
EP1738444B1 EP1738444B1 (en) 2009-04-29

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FR2867619B1 (en) 2006-06-23
DE602005014220D1 (en) 2009-06-10
FR2867619A1 (en) 2005-09-16
US20070159683A1 (en) 2007-07-12
US8035888B2 (en) 2011-10-11
EP1738444B1 (en) 2009-04-29

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